The most widely used method for preparing methacrylic acid in industry currently is the gas-phase oxidation of methacrolein with a heteropoly acid catalyst. In such method, the yield of methacrylic acid is mainly influenced by the performance of the heteropoly acid catalyst adopted, as most of the heteropoly acid catalysts contain phosphorus and molybdenum as principal components and have structures of heteropoly acid and/or salt, which are provided with low reactivity, low selectivity to methacrylic acid and short life. Therefore, great efforts have been made on the method for producing heteropoly acid catalysts, intending to improve the performance of the catalysts.
CN 1270824C has suggested to coat catalysts to increase the effective utilization of the active components of the catalyst, such that heat of reaction can be quickly removed by the increased thermal conductivity or the dilution effect from an inactive carrier, to enhance the selectivity of the catalyst. However, a catalyst with high mechanical strength is hard to obtain and the reaction temperature of which is too high to affect the life of the catalyst.
U.S. Pat. No. 5,422,326 has suggested to control the pore size distribution of the catalyst by adding organic macromolecular compounds with an average particle size of 0.01-10 μm during molding to enhance the selectivity of the catalyst. However, the reaction temperature of the catalyst is comparatively high and is not conducive for extending life of the catalyst.
CN 1874842A has suggested to obtain catalysts with enhanced performance by the following procedures: formulating a solution or slurry containing molybdenum, phosphorus and vanadium (Solution I); formulating a solution or slurry containing ammonium ions (Solution II); loading a tank (Tank A) with any one of the above solutions, Solution I or Solution II (Solution PR), and adding to a continuous liquid surface (0.01-10% area of the whole liquid surface of Solution PR loaded within Tank A) another solution (Solution LA) to formulate a mixed solution of Solution I and Solution II; drying and calcining to give the catalyst. However the catalyst also presents the disadvantage of high reaction temperature.
Therefore, there is still need to develop a catalyst with high activity and high selectivity under a relatively low reaction temperature in the reaction of synthesizing methacrylic acid (MAA) through oxidation of methacrolein (MAL).